As is well-known, implanted medical devices, such as cardiac stimulators (pacemakers, defibrillators, etc.) commonly employ an electrode lead, extending from an implanted electronic unit, in order to sense electrical activity in the patient so as to control the electrotherapy (pacing, defibrillation, antitachycardia routine, etc.) administered by the implanted stimulator to the patient. The electrode lead is typically plugged at one end into the implanted electronic unit, and has an opposite end located adjacent cardiac tissue, or in the blood stream, or attached to cardiac tissue depending on the type of activity or physiological parameter being sensed. Since this electrode lead contains one or more conductors extending from the implanted device to the sensor or electrode at the tip of the lead or along the lead, the lead itself acts as an antenna and is thus susceptible to receiving signals, such as electromagnetic interference (EMI) originating from external sources. Such interference (noise) corrupts the “true” waveform originating from the sensed electrical activity and therefore when the corrupted sensed signal is analyzed within the implanted device, in order to produce a control signal for the therapy administration, the analysis may be falsified because the noise may, for example, produce spurious data points (data outliers).
For example, theft prevention systems commonly employ high-strength magnetic fields to detect the presence of a magnetic tag placed on retail merchandise. Most of these systems modulate or pulse the magnetic field in such a way that the repetition rate falls within the pass-band of cardiac sensing amplifiers.
Conventionally, problems with electro-magnetic noise have been dealt with by removing such spurious data point by different filtering methods or by means of data processing using mathematical methods. For example, in U.S. Pat. No. 5,871,509, to Noren, a median filtering method is disclosed and in US 2006/0135886 by Lippert et al, noise is reduced or removed using a mathematical compensating method.
A number of different solutions for detecting the presence of noise have been presented within the art. In, for example, U.S. Pat. No. 7,231,251 EMI detection methods and devices for implantable medical devices are shown. A magnetic sensor, e.g. a coil, detects the presence of EMI and a noise reversion mode may be initiated in which inhibition of pacing and/or delivery of defibrillation shocks are prevented while the EMI is present. The method and device disclosed in U.S. Pat. No. 7,231,251 aims at detection of noise that may affect and disturb the sensing of intrinsic cardiac activity, which often is of a low frequency below or about 100 Hz.
However, it has been found that also impedance measurements may be distorted by EMI, particularly in a frequency spectrum of about 7-10 kHz. Cardiac impedance, for example, intracardiac impedance or transthoracic impedance, have gained a rising interest during the recent years since it has been shown that cardiac impedance provides valuable information about the mechanical functioning and mechanical work of the heart. It would hence be valuable to have a solution for detection of electro-magnetic noise that affects or disturbs measurements of cardiac impedance. Furthermore, measurements of impedance in proximity of the heart such as pulmonary impedance may also be affected or disturbed by such electro-magnetic noise. Therefore, it would also be of interest of having a solution that is capable of detecting also such pulmonary impedance disturbing noise.